92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Tuesday, 24 January 2012: 4:30 PM
Excitation of Rainfall Over the Pacific Warm Pool As Inferred From CMORPH and GHRSST
Room 245 (New Orleans Convention Center )
Richard E. Carbone, NCAR, Boulder, CO; and Y. Li

This study employs GHRSST and CMORPH merged satellite products. Analyzed in tandem, these data facilitate new insights on high frequency ocean-atmosphere interactions at small spatial scales. Conventional work on the climatology of SST and rainfall has emphasized regional scales and a coincidence of heavy rainfall with SST maxima. In contrast, here we have examined the potential role of mesoscale SST gradients in the excitation of convective rainfall events. A pivotal assumption is that the onset of rainfall events will exhibit a statistical preference for SST gradient zones if a causal relationship is likely. Such gradients are transient; exhibit a temporal scale that is daily to weekly; and have GHRSST-resolved amplitude of order 0.1-1oC/100km. Historical shipboard observations have documented gradients up to 3C on scales of order 10 km or less.

SST gradients were calculated by means of a polynomial curve fit (instead of finite differences), thereby utilizing the information from surrounding pixels, and decreasing the influence of estimate uncertainty at a specific location. A continuity algorithm was applied to CMORPH data for determination of rainfall onset locations. During the 4 year period of record (2006-2009), the influence of transient large scale disturbances was observed, including extra-tropical disturbances, Kelvin waves, equatorial Rossby waves, and westward inertio-gravity waves. Precipitation climatology exhibited four principal regimes: 1) quasi-stationary, diurnal, short-lived events; 2) long-lived, strong, westward propagating systems; 3) less organized eastward propagating systems; and 4) periods of strong suppression. When present, all regimes experienced periods of amplification associated with large scale forcings.

The presence of local SST gradients, together with a critical level and sufficient easterly shear aloft, may organize and propagate convective systems ~1000 km from their origin and contribute to a substantial fraction of total regional precipitation. Approximately 75% of major rainfall events are triggered near SST gradients. Event excitation is strongly correlated with convergence maxima inferred from the Laplacian of SST. Although the total seasonal or annual precipitation amount is strongly related to regional SST, our results show that events are triggered in a heterogeneous SST environment at slightly above average regional SST values but, generally speaking, not at SST maxima. The significance of these findings is unclear, since it is not yet known to what extent, if any, such excitation is systematic in time-space, with potential feedback to larger scales of motion.

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